A sample of mJy radio sources at 1.4 GHz in the Lynx and Hercules fields - II. Cosmic evolution of the space density of FRI radio sources

TL;DR

This study investigates the cosmic evolution of FRI radio sources up to redshift 1, revealing significant luminosity-dependent evolution and comparing their space density changes to FRIIs, enhancing understanding of radio galaxy evolution.

Contribution

It provides the first detailed analysis of the cosmic evolution of FRI radio sources, including morphological classification and comparison with FRIIs up to z ~ 1.

Findings

FRI space density increases by a factor of 5-9 by z ~ 1

Evolution appears luminosity-dependent, with lower powered sources evolving less

Results align with models of FRIIs evolution at similar luminosities

Abstract

In this paper the cosmic evolution of the space density of Fanaroff & Riley Class I (FRI) radio sources is investigated out to z ~ 1, in order to understand the origin of the differences between these and the more powerful FRIIs. High resolution radio images are presented of the best high redshift FRI candidate galaxies, drawn from two fields of the Leiden Berkeley Deep Survey, and previously defined in Rigby, Snellen & Best (2007, Paper I). Together with lower resolution radio observations (both previously published in Paper I and, for a subset of sources, also presented here) these are used to morphologically classify the sample. Sources which are clearly resolved are classified by morphology alone, whereas barely or unresolved sources were classified using a combination of morphology and flux density loss in the higher resolution data, indicative of resolved out extended emission. The space densities of the FRIs are then calculated as a function of redshift, and compared to both measurements of the local value and the behaviour of the more powerful FRIIs. The space density of FRI radio sources with luminosities (at 1.4 GHz) > 10^25 W/Hz is enhanced by a factor of 5-9 by z ~ 1, implying moderately strong evolution of this population; this enhancement is in good agreement with models of FRII evolution at the same luminosity. There are also indications that the evolution is luminosity dependent, with the lower powered sources evolving less strongly.